1. Field of the Invention
The present invention relates generally to electrical systems and devices, and more particularly to apparatus and methods for providing a multiple option select function in a water heater system.
2. Description of Related Art
Modern water heater control systems frequently use microcontrollers to handle a number of complicated gas control and safety functions. In many instances, it is desirable that a system controlled by a microcontroller be configured with certain options, such as a maximum water temperature option. Such options may be, for example, specified in a customer's order. Although such options may be not specified until an order is placed, the manufacturer may choose to substantially assemble and program an inventory of generically configured water heater control systems prior to receiving the customer order. In this business model, it may be desirable to customize the generically configured and programmed control systems just prior to shipment to satisfy customer-specified options. Such customization, for example, may include pre-selecting an option to control operation of the water heater using an option select function. Typically, the generalized software for instructing the microcontroller can be programmed to ascertain the pre-selected option and thereafter to configure the system accordingly.
One method of providing an option select function employs a configurable input/output pin on the microcontroller. As an example, a pin servicing an option select function may select the length of time a gas pre-purge blower will remain on. As another example, a pin servicing an option select function may select the maximum water temperature allowed within the water heater. Various other functional characteristics of a controlled system may be modified via a configurable pin servicing an option select function.
In the most simple implementations, a static or DC option select signal is used to select between two options. A first DC voltage (e.g., high voltage) on an option select pin represents a first option, and a second DC voltage (e.g., low voltage) on the option select pin represents a second option. A microcontroller can observe the voltage on the option select pin to determine the selected option.
Two-state static option select systems have at least two drawbacks. First, the two-state static system is limited to selection of two options. In many configurations, it may be desirable to support more than two options using a single option selector. For example, a water heater control system may be ordered with one of three or more maximum water temperature options selected. Furthermore, it may be desirable to support combinations of multiple categories of options with a single option selector. For example, a water heater control system may be ordered with various combinations of maximum water temperature options and other feature options, such as a selection of a median water temperature. To support additional options or combinations of option categories in a prior art system would typically require more than one input pin on the microcontroller. Input pins are generally in short supply in microcontrollers and thus supporting additional options or combinations of option categories in such systems would compete with other features for this limited resource (i.e., input pins).
A second drawback to prior art static two-state option select systems is that they are not fail-safe. Any circuit failure that leaves the option select signal in a high or low state would be indistinguishable by the microcontroller from a properly selected option. This result is typically undesirable, especially in water heater control systems where the option select signal may control a safety-related option. For example, a failed option select signal could result in an unexpected and dramatic increase in the water temperature, thereby introducing a risk of scalding.
To address the need for a fail-safe option select signal, an alternating signal may be used for one option, and a static signal may be used for a second option. Such a system is described in U.S. Pat. No. 5,026,270 (Adams, et al.) assigned to Honeywell, Inc.
In Adams et al., a signal on a signal option select pin controls a pilot valve trial-time circuit. The trial-time circuit determines the length of time to hold open a gas valve during an ignition attempt. Shorter trial times are obviously safer than longer trial times since there is a short period of time during which uncombusted gas may be vented, if ignition does not occur.
The invention described in Adams works by recognizing an alternating current signal having certain proper characteristics being detected on the option select pin to indicate a first option. The first option corresponds to a long trial time. Any other signal on the option select pin signifies a second option, which indicates a short trial time. Thus, the only failure that would select the longer, or less-safe option, would be a signal similar to the alternating current signal representing the first option. It is assumed in Adams, however, that random generation of the properly characterized alternating signal during a failure mode on the option select pin is highly unlikely. The circuit in Adams is therefore considered to provide fail-safe operation because the most likely failure modes cause selection of the safest option.
Solutions such as Adams, however, do not teach an option select function that provides more than two options or multiple categories of options using a single option selector. Accordingly, need exists for a method and system for providing a multiple option select function.